There are systems in which the distance and the speeds are measured by radar (microwaves), especially short-range radar. In this context, above all, small antenna arrays in compact layer building method are used. In the antenna arrays in this field having microstrip feeding, coplanar feeding or slot coupling, asymmetrical excitation is always or generally involved. In asymmetrical excitation, the signal lines (feed lines and return lines) are not developed in the same way, as in symmetrical excitation, but rather, the signal is on the feed line and the “return line” is at ground, and is usually developed as a metallic plane. In asymmetrical excitation, what may be particularly disadvantageous is the susceptibility to failure by spurious radiation from outside, which corrupts the signal.
In a large-scale integration of circuit components, because of its immunity to interference, differential, i.e. symmetrical inputs and outputs may be used. In order to be able to carry out asymmetrical feeding, in this context, costly impedance-matching sections or external baluns (balance) have to be employed. An additional disadvantage of asymmetrical excitation is radiation losses in response to a patch coupling because of the required field vector rotation of the electrical field. By patches, one is given to understand metallic radiation-emissive surfaces which are mostly rectangular.
An example of an antenna array, constructed of several layers, having asymmetrical excitation, is referred to in German patent document no. 100 63 437, in which there are two potential surfaces at ground, the so-called earth planes, each outside and parallel to the plane of stratification. Close to below the earth plane, facing the transmitting direction, which has a coupling slot, an electrical connecting section is situated. The radiation exiting from the coupling slot couples into a patch lying above it. In this context, the patch is the transmitting and/or receiving device. It is true that, in response to this screening arrangement, to a certain extent, spurious radiation from outside is deterred and radiation of the useful radiation in undesired directions is delimited, but the disadvantages caused by the asymmetrical excitation are still not satisfactorily removed.